Passivating agent for protecting the surfaces of silver-bearing materials

ABSTRACT

A SILVER PASSIVATING COMPOSITION CONSISTING OF A CARRIER INERT TO SILVER AND A THIOCARBAMIC COMPOUND.

United States Patent 3,649,373 PASSIVATING AGENT FOR PROTECTING THE SURFACES 0F SILVER-BEARIN G MATERIALS Wolfgang Dahms, Berlin, Germany, assignor to Schering AG, Berlin, Germany No Drawing. Filed Jan. 29, 1970, Ser. No. 6,957 Int. Cl. C23f 11/00 ABSTRACT OF THE DISCLOSURE A silver passivating composition consisting of a carrier inert to silver and a thiocarbamic compound.

The invention relates to a passivating agent which prevents tarnishing of silver surfaces and silver bearing surfaces.

It is known to prevent tarnishing of silver and silverbearing surfaces by means of electrolytic processes, by dipping methods and by polishing with suitable compositions. The known methods are based on producing or applying a layer which prevents contact between the silver surface with atmospheric hydrogen sulfide, the cause of the tarnishing.

It is known to employ sulfur bearing organic compounds as passivating agents.

Alkanethiols have been disclosed in US. Reissue Pat. No. 24,819 as passivating agents for silver. These compounds have limited effectiveness in protecting the metal and are sensitive to elevated temperatures. Silver contacts in electrical equipment must be stable up to about 60 to 80 C., and the known alkanethiols cannot be employed for protecting the contacts.

Ethylmercaptanes having penfluorinated substituents have been disclosed as passivating agents for silver in the German Pat. No. 1,116,216. These compounds are costly to prepare and have a limited protective elfect.

Organic disulfides have been mentioned as passivating agents for silver cleaning and polishing compositions in the published German application No. 1,243,808, but they reduce the contact resistance in a disadvantageous manner which makes their use inadvisable for passivating silver contacts in electrical equipment.

. Thiocarboxylic acids and derivatives of thiocarbonic acids have been disclosed in the German published application 1,279,428, and they can be used for protecting silverware. However, they are distinctly acidic, and do not provide useful protection when used in alkaline media. Moreover, they tend to produce hydrogen sulfide in the presence of heavy metals such as silver.

An object of this invention is the provision of passivating agents free from the afore-described shortcomings of the prior art.

Suitable passivating agents have been found to be compounds of the formula wherein R is the radical of an organic hydrocarbon radical. Representative examples of R include the phenyl and naphthyl radicals which may carry one or more substituents, or the sulfathiazolyl radical. Substituents in the phenyl or naphthyl radicals may be alkyl, such as methyl or ethyl, alkoxy, such as methoxy or ethoxy, and the like, also phenyl, nitro, halogen, such as fluorine, chlorine, or bromine, etc.

" 12laims 3,649,373 Patented Mar. 14, 1972 ice The following compounds have been found to be particularly efifective:

CsHsNH-C O-CHzSH N-phenyLmercaptoacetamide NH-C O-GHzSH N-4-methylphenyl-mercapto acetamide N-2,G-dimethylphenyl-mercaptoucetamlde N-4-methoxypl1enyl-mercaptoacetamlde N-2-pl1eny1phenyl'mercaptoacetamide Una-4: O-GHzSH N-3-nitrophenyl-mercaptoacetamide @NH-C o-omsn N-3-chlorophenylmercnptoneetamide NH-O O-GHzSH N-1-naphthyl-mercaptoacetamlde N-2-naplithyl-mercaptoacetamide N -sulfathiazolyl-thloglycolic acid "I'he passivating agents of the" invention 'are'used in aqueous or other dipping solutions or other liquid compositions in concentrations of 0.1 to and in polishing pastes and like solid or semi-solid compositions in amounts of 0.5 to

The efiiciency and the useful life of the liquid compositions may be improved if a wetting and/or emulsifying agent is added to the liquid base of water or a lower alkanol which are preferably used, and buffers are frequently useful. Calcined magnesium oxide or finely comminuted chalk may be dispersed in the liquid compositions. Liquid carriers other than water or alkanol may be employed if inert to silver.

The carriers of solid compositions may typically consist mainly of abrasives such as rouge, vienna lime or tripoli, and binders such as tallow, wax, parafiin or alcohols of high molecular weight which are chemically inert to silver.

When the otherwise conventional liquid and solid compositions additionally contain the passivating agents of the invention, the suitably treated silver or silver-bearing surfaces are protected against tarnishing even at elevated temperatures for extended periods.

The following examples are further illustrative of this invention, but it should be understood that the invention is not limited thereto.

EXAMPLE 1 A series of dipping solutions was prepared from 0.5 g. sodium diisopropyl-naphthalenesulfonate as a wetting agent, 20 g. respectively of the compounds of the invention listed above, and 1 kg. ethanol at boiling temperature. A small amount of insoluble material was removed from the solution by filtering after cooling to ambient temperature, if present. Strips of degreased sheet silver having a clean and dry surface were dipped in each solution at 20 C. for two minutes and were thereafter kept exposed to an atmosphere containing about 200 p.p.m. hydrogen sulfide at 20 C. and about 70% relative humidity for a week. When inspected after the test, they were practically free from tarnish. An untreated control sample was completely discolored.

EXAMPLE 2 A liquid silver polish was prepared by dispersing 110 g. calcined, fine-grained magnesium oxide, 10 g. carboxymethylcellulose and 5 g. N-2-naphthyl-mercaptoacetamide in 875 g. isopropanol. A piece of sheet silver polished with the dispersion in a conventional manner was far superior in its tarnishing resistance to another piece of the same material that had been polished in the same manner with a commercial silver polish containing dithioadipic acid when both specimens were dipped in a freshly prepared and filtered 2% solution of potassium sulfide for ten minutes.

EXAMPLE 3 A solid polishing paste was prepared by mixing the following ingredients with cautious heating:

G. Stearin-paraflin mixture 150 Soft montan wax 150 Tallow 100 Spindle oil 100 N -sulfathiazolylthioglycolic acid 100 Extra fine rouge 400 The paste was used with particularly beneficial results in the polishing of electrolytically deposited silver coatings since the'necessarypolishi-ng operation is combined with the passivating treatment without requiring a separate operation. When the plated and polished objects were exposed to the hydrogen sulfide atmosphere described in Example 1, they resisted tarnishing over a long period.

What is claimed is:

1. A method of protecting a silver-bearing surface from tarnishing in an atmosphere containing hydrogen sulfide which comprises applying to said surface a passivating composition essentially consisting of a carrier inert to silver and a passivating agent of the formula R in said formula being phenyl, naphthyl or-sulfathiazolyl, each of said R and R being hydrogen, lower alkyl, lower alkoxy, phenyl, nitro or halogen other than iodine, said passivating agent being present in said composition in an amount of 0.1 to 5% when said carrieris liquid, and in an amount of 0.5 to 20% when said carrier is solid.

2. A method according to claim 1, wherein in the formula R is phenyl, monoor poly-substituted phenyl, naphthyl, monoor poly-substituted naphthyl or sulfothiazoyl.

3. A method according to claim 1, wherein said agent is N-phenyl-mercaptoacetamide.

4. A method according to claim 1, wherein said agent is N-4-methylphenyl-mercaptoacetamide.

5. A method according to claim 1, wherein said agent is N-2,6-dimethylphenyl-mercaptoacetamide.

6. A method according to claim 1, wherein said agent is N-4-methoxyphenyl-mercaptoacetamide. z

7. A method according to claim 1, wherein said agent is N-3-nitrophenyl-mercaptoacetamide 1 8. A method according to claim 1, wherein said agent is N-3-chlorophenyl-mercaptoacetamide q 9. A method according to claim 1, wherein said agent is N-1-naphthyl-mercaptoacetamide.

10. A method according to claim 1, wherein said agent is N-Z-naphthyl-mercaptoacetamide.

11. A method according to claim 1, wherein said agent is N -sulfathiazolyl-thioglycolic acid. r

12. A method according to claim 1, wherein said agent is N-2-phenylphenyl-mercaptoacetamide.

References Cited UNITED STATES PATENTS 2,614,095 10/ 1952 Shelley 260-562 S OTHER REFERENCES Weiss, J. Am. Chem. Soc., vol. 69, November 1944, pp. 2684-2687.

Martin, J. Am, Chem. Soc., vol. 83, March1961, pp. 1076-1078.

Kolthofl. et aL, Volumetric Analysis, vol. III, Interscience Pub. Co., New York, 1957, pp. 391-393. g

RALPH S. KENDALL, Primary Examiner US. Cl. X.R. 

